Method for forming structures in a solar cell

ABSTRACT

A conductive contact pattern is formed on a surface of solar cell by forming a thin conductive layer over at least one lower layer of the solar cell, and ablating a majority of the thin conductive layer using a laser beam, thereby leaving behind the conductive contact pattern. The laser has a top-hat profile, enabling precision while scanning and ablating the thin layer across the surface. Heterocontact patterns are also similarly formed.

RELATED APPLICATION INFORMATION

This application is a continuation of U.S. Ser. No. 13/265,438, filedOct. 20, 2011, entitled “Method for Forming Structures in a Solar Cell”,which published Mar. 8, 2012, as U.S. Patent Publication No.2012/0055546 A1. U.S. Ser. No. 13/265,438 is a §371 National Phaseapplication of International Application No. PCT/US2010/031874, filedApr. 21, 2010, entitled “Method for Forming Structures in a Solar Cell”,and claims the benefit of previously filed U.S. Provisional Applicationentitled “Method for Forming Structures in a Solar Cell,” filed Apr. 21,2009, and assigned application No. 61/171,187; and is related to thecommonly-assigned, previously filed U.S. Provisional Applicationentitled “High-Efficiency Solar Cell Structures and Methods ofManufacture,” filed Apr. 21, 2009, and assigned application No.61/171,194; and to commonly-assigned, co-filed International PatentApplication entitled “High-Efficiency Solar Cell Structures and Methodsof Manufacture” filed Apr. 21, 2010, and assigned InternationalApplication number PCT/US2010/031869. Each of these Applications ishereby incorporated by reference herein in its entirety. All aspects ofthe present invention may be used in combination with any of thedisclosures of the above-noted Applications.

TECHNICAL FIELD

The present invention relates to solar cells. More particularly, thepresent invention relates to improved solar cell structures and methodsof their manufacture.

BACKGROUND OF THE INVENTION

The formation of metalized structures, typically fingers and bus-bars,on the front (illuminated) side of solar cells is a necessary step inmany cell designs. It is typically desirable that these structures be asfine (minimal width) as possible to minimize shading and contactrecombination losses. Heterocontact structures which can be subsequentlymetalized may also be used to reduce contact recombination losses.

SUMMARY OF THE INVENTION

The present invention addresses these requirements and others byproviding a method for the formation of front/back side metal structuresand selective heterocontact structures.

The present invention in one aspect comprises a method of forming aconductive contact pattern on a surface of solar cell, including forminga thin conductive layer over at least one lower layer of the solar cell,and ablating a majority of the thin conductive layer using a laser beam,thereby leaving behind the conductive contact pattern. A self-alignedmetallization may be formed on the conductive contact pattern.

The lower layer may include a passivation and/or antireflective layerbeneath the thin conductive layer, wherein the conductive contactpattern forms an electrical contact through the at least one lower layerto a semiconductor layer of the solar cell.

Etching or cleaning the conductive contact pattern may be used afterablating to remove residue.

In another aspect, the present invention comprises a method of forming aheterocontact pattern on a solar cell, including forming a thin layerover at least one lower layer of the solar cell; and ablating a majorityof the thin layer using a laser beam, thereby leaving behind theheterocontact pattern. Metallization may be formed over theheterocontact pattern to facilitate a conductive connection to the atleast one lower layer through the heterocontact pattern.

The thin layer may comprise multiple, different layers.

Etching the surface of the solar cell after ablating may be used to forma surface texture and/or to remove any laser ablation damage. Theheterocontact pattern may be modified by in-situ heat treatment.

All or part of the thin layer may include a doped semiconductor materialor a surface passivation layer.

The laser beam may have a top hat beam profile, and be projected througha mask. The laser beam shape created by the mask may be a regularpolygon, possibly obstructed by a thin line running across the polygon.Multiple masks or a dynamically changing mask may be used.

The laser may be operated at a wavelength and pulse width at which laserenergy is strongly absorbed in the thin conductive layer and weaklyabsorbed in the at least one lower layer. In one embodiment, more than80% of the surface area of the thin conductive layer is ablated, and thestructures remaining after said ablating are in the pattern of contactfingers and/or busbars.

Further, additional features and advantages are realized through thetechniques of the present invention. Other embodiments and aspects ofthe invention are described in detail herein and are considered a partof the claimed invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other objects, features, andadvantages of the invention are apparent from the following detaileddescription taken in combination with the accompanying drawings inwhich:

FIG. 1 is a functional block diagram of the main components of a lasermachining system, in accordance with an aspect of the present invention;

FIGS. 2a-d depict solar cell structuring using a square/rectangulartop-hat beam profile in accordance with an aspect of the presentinvention;

FIGS. 3a-d depict solar cell structuring using a square-with-linetop-hat beam profile in accordance with an aspect of the presentinvention;

FIGS. 4a-e depict solar cell structuring using combining two differentmask shapes in accordance with an aspect of the present invention; and

FIG. 5 depicts a completed finger/bus-bar structure on the front face ofa solar cell created according to an aspect of the present invention;

FIGS. 6a-b show an exemplary Ti solar cell finger in accordance with thepresent invention;

FIG. 7 shows an exemplary Ni solar cell finger in accordance with thepresent invention; and

FIGS. 8a-b show exemplary self-aligned metallization of the finger ofFIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides, in one aspect, a manufacturing approachfor the production of solar cell structures using negative laserablation to form line structures on the front (and/or rear) face of asolar cell, resulting in very fine features which minimize interferencewith incident light and provide other advantages.

Cell structuring is performed by, e.g., laser direct write using mask(or mask-less) projected “top hat” homogenous beam profiles such that anegative of the required line pattern is “written” or ablated, by anincident laser. Homogeneous top hat profiles allow for controlled thinfilm ablation with minimal pattern overlap and high resolution. Suchpatterning is not typically possible with gaussian beam profile systems.

FIG. 1 is a functional diagram showing the main components of a systemfor performing laser ablation using masked, projected top hat homogenousbeam profiles. This exemplary system 10 includes a laser source 12,homogenizer and optics 14, mask 16, scanner 18 (including a lens in oneembodiment), and translation stage 20 for holding a solar cell 22requiring ablation.

The formation of top-hat laser profiles (e.g., known to be a controlledflat top profile rather than Gaussian) can be effected using very highpower (>300 W) lasers combined with homogenizers, masks, mirrors,translation stages and/or scanners to enable full area exposure anddirect writing of repetitive features, with the machined features beingdefined by the mask, translation stage, and/or scanner. Laser sourcesused may be high power multimode sources. The laser source wavelength,pulse width, repetition rate, and pulse energy are chosen to best suitthe process requirements. Examples of such laser sources include diodepumped solid state Nd:YAG and Excimer lasers. Other examples includepulsed (Q-Switched) lasers or continuous wave lasers. The laser may beoperated at a wavelength and pulse width at which laser energy isstrongly absorbed in the thin film layer and weakly absorbed in thesemiconductor substrate, to effect ablation of the upper layer.

The exact size and shape of the projected beam which writes the patternis determined by the system optics and the mask to match the processrequirements. In general, and without limitation, the laser beam shapecreated by the mask may be any regular polygon, possibly obstructed by athin line running across the polygon. Multiple masks or a dynamicallychanging mask may be used to process a single solar cell. The laser maybe a pulsed laser, have a top hat beam profile, process an area with asingle laser pulse, where the overlap of the scanned beam is less than20% of the single pulse processing area. In accordance with theinvention, more than 80% of the surface area of the upper thin filmlater may be ablated, leaving behind the requisite fine pattern.

Two exemplary shapes and scanning patterns which can be used to remove athin film from the majority front of a solar cell are shown in FIGS. 2and 3 in accordance with the present invention.

As one example, FIGS. 2a-d show a simple square or rectangle exposureunit 132 (FIG. 2d ) which is moved along translation pattern 130 (FIG.2c ). The fine structure in the resultant thin film 126 is formed bynegative laser ablation during appropriate translation of the beamand/or solar cell relative to each other. More particularly, a substrate122 (shown in side view of FIG. 2a —and possibly including other layers)has formed thereon a thin film layer 124 requiring ablation. The laserexposure unit 132 is translated/stepped/scanned in a planar translationpattern 130 (FIG. 2c ) over the film, avoiding areas 140, resulting in afinely patterned structure 126 (FIG. 2b ).

As another example, FIGS. 3a-d show a square or rectangle exposure unit232 (FIG. 3d ) which has a central blocked segment 233 in the mask whichis moved along translation pattern 230 (FIG. 3c ). This blocked segmentcan be used to form the fine structure 226 in the thin film by negativeablation. More particularly, a substrate 222 (shown in side view of FIG.3a —and possibly including other layers) has formed thereon a thin filmlayer 224 requiring ablation. The laser exposure unit 232 with blockedarea 233 is translated/stepped/scanned in a planar translation pattern230 (FIG. 3c ) over the film, blocking areas 240, resulting in a finelypatterned structure 226 (FIG. 3b ).

The exposure unit shapes illustrated in FIGS. 2d and 3d can be combinedinto a single laser scan sequence, as shown in FIGS. 4a-e . Moreparticularly, a substrate 322 (shown in side view of FIG. 4a —andpossibly including other layers) has formed thereon a thin film 324requiring ablation. The laser exposure unit 332 with blocked area 333(FIG. 4d ), and also rectangular pattern 334 (FIG. 4e ), aretranslated/stepped/scanned in a planar translation pattern 330 (FIG. 4c) over the film, avoiding areas 340, resulting in a finely patternedstructure 326 (FIG. 4b ). Very rapid changing of the mask shapes 332 and334 may be achieved by, for example, using a high speed galvanometerwhich inserts a beam blocking element at (or very close to) the plane ofthe mask. Therefore, transition between the shapes shown in FIGS. 2 and3 may be done very quickly (in milliseconds) without adversely affectingthe overall processing time. Such dynamic mask changing techniques canbe synchronized with beam and/or stage translations to provide moreflexibility in the types of patterns which can be economically produced.

FIG. 5 shows a solar cell 422 having a pattern of fingers 426 and busbars 427 on a surface thereof, formed in accordance with any of theabove-described aspects of the present invention. The resultingstructures can be very fine (<10 um line width). The minimum patternresolution is nominally defined by the optical resolution of the system.With this technique many types of thin films may be cost effectivelypatterned to form solar cell structures. As an example consider a thinfilm with an ablation threshold of 3 J/cm². A 300 W laser source wouldbe capable of patterning the entire area of a 250 cm² solar cell in aslittle as 2.5 seconds. Such high throughput supports the demandingeconomics of solar cell production.

One improvement involves using any form of texture on the substrate(122, 222, 322) where this texture is of a scale comparable to that ofspatial inhomogeneities present in the intensity profile of the top hatlaser beam. Such texture provides an important beam-homogenizingfunction—as it is generally desirable to operate the laser beam with afluence at or close to the ablation threshold of the layer (124, 224,324) and hence, a locally homogenized beam adds a greater deal ofcontrol and precision.

The present invention has been described as applying to the front, lightreceiving, surface of a solar cell but can be equally applied tostructures on the rear surface. Application of the invention to bothsides of a solar cell will result in a bifacial solar cell which couldbe capable converting photons incident on both front and rear surfaces.

With reference to the above figures, the present invention provides amethod for forming, e.g., contact metallization or other fine structuresin a solar cell, the method including the steps of:

A1) On the front (or back) surface of a solar cell (e.g., semiconductorsubstrate) a continuous thin film metal layer (124, 224, 324) isdeposited, the metal film material and thickness being selected suchthat it has desirable adhesion, contact resistance, plating and laserablation properties. Examples of metals or metal alloys which may havesuitable properties include: nickel, nickel-vanadium, nickel-niobium,nickel-tantalum, aluminum, aluminum-silicon, chromium, titanium, silverand tungsten.

A2) By laser ablation the thin film metal layer is removed from themajority of the front surface leaving behind structures (126, 226, 326)for example fingers or bus-bars, which can form the front contactmetallization for the solar cell.

A3) Self-aligned metallization of these thin film metal structures canoptionally be performed. Nominally this is done to reduce seriesresistance losses. Typically this metallization is performed byselective electroplating, light-induced-plating and/or electrolessplating onto the existing thin film metal structures.

In addition to the steps A1) to A3) listed above, many variations andadditions are possible in accordance with the present invention. Forexample after step A2) a chemical clean may be performed to remove anyresidual metal. Further a thermal anneal step may be performed prior toself-aligned metallization in step A3). In some embodiments, surfacepassivation and/or anti-reflective layers can be deposited on the solarcell prior to deposition of the thin film metal layer in step A1). Suchpassivation and anti-reflection layers could allow, or be modifiedin-situ, for electrical contact with the underlying semiconductorsubstrate. Alternatively or further in some embodiments surfacepassivation and/or antireflective layers may be deposited on the solarcell at some stage after final metallization in step A3).

Another improvement involves the use of multi-layered metal as the thinlayer (124,224,324). A single metal (or metal-alloy) layer may notprovide optimal process or solar cell device performance. By using amulti-layered metal stack different metal properties may be exploited toprovide better performance. For example in a two layer metal stack it isdesirable that the bottom metal (next to substrate 122, 222, 322) have alow ablation threshold while the top metal be compatible with subsequentself aligned metallization steps (A3). An example of such a stack couldbe Ni as a top metal over Ti as a bottom metal.

With reference to the above figures, another embodiment of the presentinvention involves a method for forming selective heterocontact (or ascould otherwise more generally be described as selective emitter)structures in a solar cell, the method includes the steps of:

B1) On the front surface of a semiconductor substrate a continuous thinfilm layer (124, 224, 324) is deposited. This thin film layer mayconsist of multiple stacked films, the film stack being designed withdesirable contact resistance, contact recombination, series resistance,laser ablation, adhesion and etch resistance properties.

Films used in this stack may include:

doped semiconductor films such as polycrystalline or amorphous silicon,silicon, polycrystalline or amorphous silicon-carbide, polycrystallineor amorphous diamond-like-carbon; and/or

undoped films such as silicon oxide, silicon nitride, intrinsicamorphous silicon, intrinsic polycrystalline silicon, aluminum oxide,aluminum nitride, phosphorus nitride, titanium nitride, intrinsicamorphous silicon carbide, intrinsic polycrystalline silicon carbide.

B2) By laser ablation the thin film layer is removed from the majorityof the front surface leaving behind structures (126, 226, 326), forexample fingers and bus-bars. These structures can subsequently bemetalized.

B3) The front surface of the cell may be etched to form a surfacetexture and/or to remove any laser ablation damage, the laser patternedthin film structures having been designed to be resistant to this etchwill remain.

B4) Self-aligned metallization of these thin film structures may beperformed. Typically this metallization is performed by selectiveelectroplating, light-induced-plating, and/or electroless plating ontothe existing thin film structures.

In addition to the steps B1) to B4) listed above, many variations andadditions are possible. For example after step B3) a dopant diffusionand/or thermal oxidation may be performed in order to form an emitterand/or effectively passivate the front surface of the cell. This has theadditional advantage that the thermal oxide can aid in the selectiveplating of thin film structures. Another variation may involve theaddition of a thermal treatment after step B2) to change properties,such as etch resistance, of the thin film layer. Typically in allschemes an anti-reflection and/or additional passivation layer may bedeposited at some stage after final metallization in step B4).

The invention can be applied to many solar cell structures, includingany of those listed in the above-incorporated Patent Applications. Thefollowing are merely examples, but the invention is not limited to theseexamples.

EXAMPLE 1

A thin layer of Titanium metal is ablated from a passivated and randompyramid textured mono-crystalline silicon wafer to form thin metalfingers. Titanium is deposited on the front of the wafer by physicalvapor deposition to a thickness of approximately 500 Å. This metal isablated from the majority of the wafer surface to leave behind thinmetal fingers with a width of approximately 50 μm. Such metal ablationcan be performed using an Nd:YAG laser. When such a laser is imagedthrough a mask and a top-hat (or pseudo top-hat) profile created thensingle shot ablation with minimal shot overlap (<20%) can be used tocreate fine line metal features which form the finger and bus-barpattern of the solar cell. In this example an industrial laser systemcapable of delivering 140 W to the sample can process a 250 cm² solarcell in approximately 4 sec. Further it can be shown, by cell lifetimemeasurements before and after laser ablation, that the requiredoperating fluence is below the damage threshold of the cell passivation.An exemplary solar cell finger created by this large area laser ablationis depicted in FIGS. 6a -b.

EXAMPLE 2

A thin layer is Nickel-7% Vanadium metal alloy is ablated from apassivated and random pyramid textured mono-crystalline silicon to formthin metal fingers. Ni(7% V) is deposited by physical vapor depositionand to a thickness of approximately 500 Å. This metal is ablated fromthe majority of the wafer surface to leave behind thin metal fingerswith a width of approximately 50 μm. Such metal ablation can beperformed using an Nd:YAG laser. When such a laser is imaged through amask and a top-hat (or pseudo top-hat) profile created then two shot perarea can be used to create fine line metal features which form thefinger and bus-bar pattern for a solar cell. Further such a patternednickel film can directly act as a suitable seed layer for self-alignedmetallization. Such metallization of the patterned nickel seed can, forexample, be performed by electro-plating of Ni to a thickness of ˜1 μmfollowed by electro-plating of Cu to a thickness of approximately 20 μm.The resulting completed metal stack can thus have sufficiently highelectrical conductance to function as front metal fingers with lowseries resistance power loss. An exemplary solar cell finger created bythis large area laser ablation is depicted in FIG. 7; and theself-aligned metallization is shown in FIGS. 8a -b.

One or more of the process control aspects of the present invention canbe included in an article of manufacture (e.g., one or more computerprogram products) having, for instance, computer usable media. The mediahas embodied therein, for instance, computer readable program code meansfor providing and facilitating the techniques of the present invention.The article of manufacture can be included as a part of a computersystem or sold separately.

Additionally, at least one program storage device readable by a machineembodying at least one program of instructions executable by the machineto perform the capabilities of the present invention can be provided.

The flow diagrams and steps disclosed herein are just examples. Theremay be many variations to these diagrams or the steps (or operations)described therein without departing from the spirit of the invention.For instance, the steps may be performed in a differing order, or stepsmay be added, deleted or modified. All of these variations areconsidered a part of the claimed invention.

Although preferred embodiments have been depicted and described indetail herein, it will be apparent to those skilled in the relevant artthat various modifications, additions, substitutions and the like can bemade without departing from the spirit of the invention and these aretherefore considered to be within the scope of the invention as definedin the following claims.

What is claimed is:
 1. A method comprising: forming a contact pattern ona surface of a solar cell, the solar cell comprising a texturedsemiconductor substrate, the forming comprising: forming at least onefilm layer over a lower layer of the solar cell, the at least one filmlayer comprising a material having an ablation threshold; irradiating afirst portion of the at least one film layer with a laser, withoutirradiating a second portion of the at least one film layer, the laserhaving an at least partially homogenized beam profile, and theirradiating ablating the entire first portion of the at least one filmlayer, leaving the second portion of the at least one film layer as thecontact pattern, the laser being operated at a fluence near or at theablation threshold of the material of the at least one film layer; andwherein during the irradiating, the textured semiconductor substratefurther homogenizes the at least partially homogenized beam profile tofacilitate operating the laser at the fluence near or at the ablationthreshold of the material of the at least one film layer.
 2. The methodof claim 1, wherein the at least partially homogenized beam profilecomprises a non-Gaussian beam profile.
 3. The method of claim 2, whereinthe non-Gaussian beam profile comprises a non-Gaussian top hat beamprofile.
 4. The method of claim 1, wherein the textured semiconductorsubstrate comprises a texture having a size scale comparable to one ormore spatial inhomogeneities in the at least partially homogenized beamprofile.
 5. The method of claim 4, wherein the textured semiconductorsubstrate comprises a random pyramid-shaped texture.
 6. The method ofclaim 1, further comprising forming self-aligned metallization on thecontact pattern.
 7. The method of claim 1, wherein the material of theat least one film layer comprises a conductive material, and the contactpattern is a conductive contact pattern.
 8. The method of claim 7,wherein the at least one film layer comprises at least one film layer ofa plurality of film layers, the plurality of film layers comprising twoor more different conductive materials.
 9. The method of claim 7,wherein the conductive contact pattern comprises a plurality ofconductive fingers having a width of approximately 50 μm or less. 10.The method of claim 1, wherein the material of the at least one filmlayer comprises a semiconductor material, and the contact patterncomprises a hetero-contact pattern.
 11. The method of claim 10, whereinthe semiconductor material further comprises a conductive dopant. 12.The method of claim 10, wherein the at least one film layer comprises atleast one film layer of a plurality of film layers, the plurality offilm layers comprising two or more different semiconductor materials.13. The method of claim 10, wherein the hetero-contact pattern comprisesa plurality of at least partially conductive fingers having a width ofapproximately 50 μm or less.
 14. The method of claim 1, wherein theirradiating comprises projecting the at least partially homogenizedlaser beam through a mask.
 15. The method of claim 14, wherein the maskshapes the laser beam into a beam with a regular polygon cross-section.16. The method of claim 14, wherein mask shapes the laser beam into abeam with a regular polygon cross-section with a thin obstructed linerunning across the regular polygon cross-section.
 17. The method ofclaim 14, wherein the irradiating comprises projecting the at leastpartially homogenized laser beam through multiple masks or a dynamicallychanging mask.
 18. The method of claim 1, wherein the first portion ofthe film layer ablated by the at least partially homogenized laser beamcomprises more than 80% of the film layer.